As a next generation communication standard that is compatible with a Long Term Evolution (LTE) system, the 3rd Generation Partnership Project (3GPP), which is the standardization organization, is promoting the standardization of Long Term Evolution Advanced (LTE-Advanced: LTE-A). In the LTE system, a wireless communication device (hereinafter, also referred to as “Network Entity (NE)”) of a network (Evolved Universal Terrestrial Radio Access Network: E-UTRAN) provides one or more communication cells.
The wireless communication device is a device such as a wireless communication base station (E-UTRAN NodeB: eNB), a remote base station (Remote Radio Head: RRH), or a relay device (relay node or repeater), which serves as an access point of a wireless communication terminal (User Equipment: UE). The wireless communication terminal belongs to one communication cell among one or more communication cells provided by the wireless communication device. In addition, the wireless communication terminal may use a plurality of frequencies and belong to a plurality of communication cells. Further, the wireless communication terminal may perform transmission and reception with a plurality of communication cells of one frequency. Hereinafter, the wireless communication device is referred to as a “base station”, the wireless communication terminal is referred to as a “terminal”, and the communication cell is referred to as a “cell”.
In the LTE system, a DL grant (also referred to as DL assignment) instructing data assignment of DownLink (DL) to the terminal from the base station and a UL grant instructing data assignment of UpLink (UP) to the base station from the terminal are transmitted, using a Physical Downlink Control Channel (PDCCH). The DL grant notifies a DL resource allocated to a terminal, in a subframe to which the DL grant is transmitted. On the other hand, the UL grant has different subframes for a resource allocated to a terminal in a Frequency Division Duplex (FDD) system and a Time Division Duplex (TDD) system. In the FDD system, the UL grant notifies a UL resource allocated to a terminal, in a target subframe after 4 subframes from the subframe to which the UL grant is transmitted. Further, in the TDD system, the UL grant notifies a UL resource allocated to a terminal, in a target subframe after 4 or more subframes from the subframe to which the UL grant is transmitted. In the TDD system, as the allocation target subframe for the terminal, after how many subframes from the subframe, to which the UL grant is transmitted, a subframe is allocated is determined in response to a pattern in which an uplink line and a downlink line are time-divided (hereinafter, referred to as “UL/DL configuration pattern”). However, even in any UL/DL configuration pattern, the UL subframe is the subframe after 4 or more subframes from the subframe to which the UL grant is transmitted.
The terminal performs blind decoding (BD) on the common search space which is in the PDCCH region and the UE specific search space and knows whether or not a control signal required by the terminal is transmitted. The control signals for all terminals are transmitted in the common search space, and the control signal for each terminal is transmitted in the UE specific search space.
In the LTE-Advanced system, with the increase in the number of terminals to be connected to a single cell or the increase in the amount of communication packets at each terminal, there is concern of resource shortage of a PDCCH region. If the base station cannot map to the PDCCH, a control signal instructing a data assignment for the terminal due to the shortage of resources of a PDCCH region, the data assignment for the terminal is not performed. In this case, even if the resources of a Physical Downlink Shared Channel (PDSCH) region to which data is mapped is not occupied, the resources cannot be used, and thus there is a concern that the system throughput is reduced.
As a method to resolve the resource shortage in the PDCCH region, it is considered that a control signal addressed to the terminal under the base station is placed also in the resource used as a data region (for example, a PDSCH region). A new region, to which the control signal addressed to the terminal under the base station is mapped, is referred to as an Enhanced PDCCH (ePDCCH) region, a New-POOCH (N-PDCCH) region, an X-PDCCH region, or the like. Further, a relay technology is introduced to the LTE-Advanced system, and the control signal for a relay device is placed in the data region. Since there is a possibility that the control signal for the relay device is extended and used as the control signal for the terminal, the region in which the control signal is placed is also referred to as an R-PDCCH (POOCH for a relay device). It is possible to increase the number of control signals by increasing the number of the new regions in which the control signal is placed as described above. Further, by changing between cells a new region (hereinafter, also referred to as “ePDCCH”) other than the PDCCH region to which the control signal is transmitted, a transmission power control for the control signal transmitted to a terminal located in the vicinity of the cell edge, an interference control of applying a transmitted control signal to other cells, or an interference control of applying the control signal from other cells to its own cell can be realized.
In the LTE-Advanced system, a region (R-PDCCH region) in which the control signal for the relay device is placed is provided in the data region (for example, PDSCH region). The DL grant and the UL grant are placed also in the R-PDCCH similarly to the PDCCH. Further, in the R-PDCCH, the DL grant is placed in a first slot and the UL grant is placed in a second slot (see NPL 1). Since the decoding delay of the DL grant is reduced by placing the DL grant only in the first slot, the relay device can be ready for the transmission of an ACK/NACK for the DL data (transmitted after 4 subframes from receiving the DL grant, in the FDD). If “relay device subframe configuration (RN subframe Config)” is transmitted from the base station in a Radio Resource Control (RRC) layer, the relay device determines the allocated resource (search space) of the R-PDCCH region based on the R-PDCCH configuration (rpdcch Config) included in the configuration information. Further, the relay device receives a control signal transmitted from the base station in the R-PDCCH region.
[Description of Resource]
In the LTE system and the LTE-Advanced system, one Resource Block (RB) is “12 subcarriers×0.5 msec”, and a unit of a combination of two RBs on a time axis is termed a RB pair. Accordingly, the RB pair is “12 subcarriers×1 msec”. In a case of representing a set of 12 subcarriers on the frequency axis, the RB pair is simply termed a RB. Further, the RB pair is termed a Physical RB (PRB) pair in a physical layer. Further, a unit of “one subcarrier×one OFDM symbol” is termed a Resource Element (RE). The number of OFDM symbols per one RB pair changes depends on the length of the Cyclic Prefix (CP) of an OFDM symbol. The number of REs of the region, in which the ePDCCH is placed, per one RB pair is different depending on the number of OFDM symbols or the number of REs used for the Reference Signal (RS). However, the number of OFDM symbols which can be used and the reference signal change in each subframe. Accordingly, the reception quality of the ePDCCH is reduced in the subframe having a small number of REs which can be used. Further, in a case where the PDCCH region is not used for the ePDCCH, the number of OFDM symbols which can be used in the ePDCCH is reduced depending on the number of OFDM symbols in the PDCCH region. The number of OFDM symbols used in PDCCH is 1 to 4.
[Description of Reference Signals]
The reference signal has the following main three types. The first type is a Cell specific Reference Signal (CRS). The CRS is transmitted using a specific RE in all RBs. However, the CRS is transmitted also in the data region in subframes other than a Multimedia Broadcast multicast service Single Frequency Network (MBSFN) subframe, but is transmitted only in the first two OFDM symbols in the MBSFN subframe. Further, in the CRS, the RE to be placed is determined by the cell ID.
The second type is a UE specific Reference Signal (DMRS). The DMRS is transmitted for decoding the PDSCH. The antenna port of the DMRS to be dynamically used in the DL assignment is indicated to the terminal. Consequently, the base station can indicate different numbers of DMRS for each terminal. Further, the DMRS is transmitted in the data region (see NPL 2).
The third type is a Channel State Information Reference Signal (CSI-RS). In the CSI-RS, although the resource that can be transmitted by the base station is defined in advance, it is possible to change the resource to be actually transmitted, in each cell. The terminal can know an interval and a resource to be transmitted, based on the specific control information transmitted in a RRC layer.
Further, the PDCCH and the R-PDCCH have four levels including levels 1, 2, 4 and 8 as an aggregation level (for example, see NPL 2). Then, the levels 1, 2, 4 and 8 respectively have search spaces which are configured from 6, 6, 2 and 2 types of mapping candidate positions. Here, the mapping candidate positions are candidates of regions to which control signals are mapped. If one aggregation level is configured for one terminal, a control signal is actually mapped to one of the mapping candidate positions of a plurality of control signals that the aggregation level has.
FIG. 19 is a diagram showing an example of a search space corresponding to the R-PDCCH. Each ellipse shown in FIG. 19 shows the mapping candidate position of the control signal of each aggregation level. A plurality of mapping candidate positions in each search space of each aggregation level are continuously placed in Virtual Resource Blocks (VRB). Then, each mapping candidate position in the VRBs is mapped to Physical Resource Blocks (PRB) by signaling of the RRC layer.
It is considered that the search space corresponding to the ePDCCH is individually configured for each terminal. With regard to the design of the ePDCCH, it is possible to use a portion of the design of the R-PDCCH, or to use a design totally different from the design of the R-PDCCH. Actually, it is considered that the designs of the ePDCCH and the R-PDCCH are different.
As described above, in the R-PDCCH, a DL grant is mapped to a first slot and a UL grant is mapped to a second slot. In other words, the resource to which the DL grant is mapped and the resource to which the UL grant is mapped are divided in the time axis. In contrast, it is considered that in the ePDCCH, the resource to which the DL grant is mapped and the resource to which the UL grant is mapped are divided in the frequency axis (that is, subcarriers or RB pairs) or an RE in a PRB pair is divided into a plurality of groups.
Further, as a placing method of the ePDCCH, both of “Localized allocation” which collectively places the ePDCCHs in close positions on the frequency band and “Distributed allocation” which distributes and places the ePDCCHs on the frequency band are considered. “Localized Allocation” is an allocation method which obtains a frequency scheduling gain and can allocate the ePDCCH in a resource having good channel quality, based on channel quality information. “Distributed Allocation” is able to distribute the ePDCCHs on the frequency axis so as to obtain a frequency diversity gain. Further, it is considered that the base station simultaneously configures “Localized search space” and “Distributed search space”.